The present applicants have already proposed a method for acellularizing an animal tissue, such as pericardium or tendon, harvested from an animal, such as a cow or a pig, in order to transplant the animal tissue into a human body (see Patent Literature 1 and the like). Here, the biological tissue harvested from an animal and acellularized (hereinafter referred to as “acellularized tissue”) may not be used immediately after the acellularization but sterilized for storage for the time being. In order to put such an animal-derived acellularized tissue to practical use, treatment for sterilizing the acellularized tissue is essential.